Various environmental energy sources such as light, heat, vibrations and RF may be utilized for supplying power to a load such as a wireless sensor network. The wireless sensor network may have a problem in maintenance such as the exchange of a battery. Therefore, if energy harvesting which utilizes environmental energy is used, the wireless sensor network may be operated with an infinite life. Among various kinds of energy harvesting techniques, light energy harvesting has the best capability since its energy source is abundant and can be easily obtained. However, since the magnitude of available energy source is very small when using indoor light energy or being used in a dark day, an energy harvesting circuit should be designed to use a low power, and an existing energy harvesting technique should be improved.
The light energy harvesting system needs a voltage converter for converting a low input voltage obtained from a solar cell, which plays a role of energy harvester (hereinafter, referred to as a “harvester”), into a voltage of a high level, which may be utilized by a load. In case of an inductive DC-DC converter using an inductor, since an external inductor is used, there are drawbacks in costs and size.
The maximum power point tracking (hereinafter, referred to as “MPPT”) technique represents a technique of controlling an input impedance so that an output power of a harvester becomes a maximum power point (hereinafter, referred to as “MPP”). Generally, the MPPT technique should be applied to an energy harvesting circuit receiving power from a harvester.
Since a charge pump connected to a harvester with a specific curve has a separate current detector and a control signal generating unit in order to transfer an optimum power to a storage medium such as a capacitor and a battery, power of the current detector is wasted in proportion to the output current. In addition, the MPPT technique should be designed with a very low power since it is always operating. Moreover, a separate voltage adjuster is required for converting a generated output voltage to a fixed voltage necessary to a load, which causes another loss.